We describe here in more detail, for those who are interested, how our experimental study was done.

The three chief results that we have found are that inhibitory receptors are far denser in the dorsal thalamus than in any other of major subdivisions of the thalamus in 9 widely differing vertebrate species. The constancy of the relatively dense binding in the dorsal thalamus across the nine species suggests that the presence of receptors for the inhibitory neurotransmitter GABA may be a fundamental principal of neural organization among vertebrates.

In this table, symbols indicating the relative degree of attachment of an inhibitory neurotransmitter (GABA as shown by muscimol binding) in the four subdivisions and the total thalamus for the seven mammals and two non-mammals.  In each case the greatest degree of binding was found in the dorsal thalamus. Since this structure sends and receives information from the neocortex, it is at this level that the impulses can be inhibited.  Note the difference between the average (mean) amount of inhibitory neurotransmitter in the mammals versus the non-mammals in the dorsal thalamus.  The greater amount found in the mammals suggests a systematic increase in the binding of the inhibitory neurotransmitter GABA over geological time.

Second,  the pattern of the binding across the thalamic subdivisions was similar regardless of the species' phylogenetic position (i.e., neither it’s taxonomic class, subclass, order, or family made any difference in the pattern of labeling).  This finding implies that there is a neurochemical 'conservation' over long periods of brain evolution.

Because reliable mammal/non-mammal differences and reliable marsupial/placental differences were found in the relative density of muscimol binding in dorsal thalamus, we were encouraged to analyze the possibility of a phylogenetic trend along the Anthropoid ancestral lineage.  The figure above shows the amount of muscimol binding among the species when they are arranged on the basis of their nearness of kinship, or recency of last common ancestor, with Anthropoids and thus with humans. For the entire group of species, no reliable trend appears.  However, when the 5 placental mammals alone (omit the two marsupials and the two non-mammals) are analyzed a statistically reliable trend in muscimol binding appears in dorsal thalamus (see solid line in Figure, left). Therefore, an increase in GABA (as shown by muscimol binding) in placentals is probably present. This would likely have continued into the human lineage.

And finally, the increase of muscimol binding in mammals compared to non-mammals suggests a greater role of inhibitory processing in the mammalian dorsal thalamus,  That is, there has been a systematic increase in the inhibitory neurotransmitter GABA over geological time.  This difference depends on the degree of kinship with Anthropoids.  Because there are close parallels in the evolutionary development of dorsal thalamus and cerebral neocortex, a difference in the thalamus in mammals might well be a concomitant of the enhanced cortical development of the cerebrum seen in the mammals over the same period of geological time -- perhaps a demand placed on the dorsal thalamus by the emergence of mammalian cortex itself.  That may mean for human evolution there has been a progressive increase not only in cortex but also in the thalamus supporting the cortical development.   That is, along with selective pressure for an increase in cortical size has come an increase in the inhibitory receptors and therefore the ability to inhibit ongoing activity.
 

The Authors

Karen Glendenning is a professor of Neuroscience at Florida State University. She has published 49 articles and 3 book chapters. She has given 43 talks at scientific meetings and 18 invited colloquia.   She was funded by grants from National Institutes of Mental Health for over 25 years to conduct this and other research studies.  (Grants NS07726 and DC00197).